The marine environment must be regarded as a great potential source of physiologically active chemicals, yet few chemists have turned their attention to this field. The observations of marine biologists point to the existance of chemicals which ensure the survival of marine organisms by preventing the growth of marine bacteria. In general, these chemical defense systems are found among the more primitive organisms. These organisms should therefore be investigated as potential sources of useful antibiotics. Marine invertebrates which feed by filtering microscopic organisms, including bacteria, must continually produce antibacterial compounds. We have therefore screened extracts from sponges and ascidians and found several compounds which show wide spectrum activity in vitro. In an investigation of the chemical defense system of the Sea Hare we have traced antibacterial compounds to their algal sources. We propose to screen a number of marine organisms for antibiotic activity. We will isolate the active compounds for structural elucidation and pharmacological evaluation. Potentially useful compounds will be synthesized to provide material for further studies.